1. Technical Field
The present invention relates to telephone privacy systems and, more particularly, to systems which automatically distinguish authorized callers from unauthorized callers.
2. Discussion of the Prior Art
Telephone privacy and screening systems are well known in the prior art. The primary utilization of such systems is to inform a telephone subscriber or user that an authorized caller is attempting to call, in which case the subscriber can answer the call or not as he or she sees fit. Some prior art systems, such as that disclosed in U.S. Pat. No. 3,514,548 (DeMeo), inhibit ringing at the subscriber's transmitter/receiver set unless an authorized or desired call is being received. The DeMeo system responds to all calls by mechanically lifting the system user's handset off its cradle. If the caller enters the proper code, the phone is caused to ring, notifying the subsciber or system user that an authorized caller is on the line. If a proper code is not dialed by the caller, the handset is returned to the on-hook position after a pre-set time interval. While this system performs its intended function, it responds to only one caller code and is therefore able to distinguish between authorized and unauthorized callers but cannot identify individual authorized callers. Moreover, since the handset is lifted off hook without further response on the telephone line, an unauthorized caller is made aware that some sort of screening process may be going on and may enter the appropriate code by a trial and error process. Further, if an authorized call is received when the subscriber is away from the phone or otherwise unable to answer, there is no way to record the identity of the authorized caller so that the call may be returned. Finally, since the subscriber phone in the DeMeo system does not ring in response to unauthorized calls, the subscriber is unaware of such calls and is not provided with the option of answering them.
In the system disclosed in U.S. Pat. No. 4,266,098 (Novak), different codes are assigned to individual authorized callers, and the caller's name is displayed visually if his or her code is detected after reception of an incoming call. However, the subscriber's phone does not ring unless an authorized caller code is detected, thereby removing the subscriber's option of answering calls from unauthorized callers. Also, the subscriber must look at the visual display to determine the caller's identity and therefore must divert his or her attention from his or her present task. Further, in the Novak system, the code entry request is in the form of a recorded voice message which invites the caller to enter an authorization code. Unauthorized callers may, by trial and error, enter an authorized code and thereby gain unauthorized access to the subscriber.
Another telephone privacy system is disclosed in U.S. Pat. No. 4,074,078 (Jansen). That system has no means for distinguishing individual authorized callers from one another and does not produce a ringing in the subscriber's phone unless a correct authorization code is detected.
Other related types of privacy systems may be found in U.S. Pat. Nos. 4,369,339 (Castro et al), 3,787,626 (Subieta), 3,904,830 (Every et al), and 3,784,721 (Kilby). None of these prior art systems discriminates between authorized callers without requiring the subscriber to divert his or her attention; nor do the prior art systems perform a screening operation without making it evident to an unauthorized subscriber that such operation is being performed.